Long distance telecommunications requires amplification and often involves multiplexing. These operations are facilitated if the two directions of transmission are isolated from one another. Isolation is obtained by providing four wires: two for transmission in each direction. While long distance lines are generally implemented over four wire lines, virtually all subscriber loops are implemented with a single pair of wires which carries transmissions in both directions. Hybrid circuits are provided to allow interconnection of four wire trunks with two-wire lines. If the impedance of the two-wire line matches that of the four-wire line, all energy incoming to the two-wire line from the four-wire line is coupled to the two-wire line and none is reflected back on the four-wire line as an echo. However, circuits are generally switched connections so that the impedance changes with each connection. Because of this, echoes on the four-wire line are a problem. To control echoes, echo cancellers are employed. An echo canceller is positioned at the hybrid circuit and subtracts a properly delayed and attenuated copy of a signal received from the far end of the four-wire line from the signal propagating from the near end of the four-wire line at the hybrid circuit back toward the far end of the line in order to cancel echo components.
An echo canceller, while invaluable for voice communications, may destroy data in full duplex data communications. Consequently, it is desirable to disable an echo canceller in a circuit during data communications. To facilitate this, "called" modems are designed to generate an answer tone (i.e., a constant single frequency signal) at the start of their transmissions. The International Telecommunications Union (ITU) developed two standards in this regard: the G.164 standard and the more recent G.165 standard, which is the currently recommended standard for echo cancellers. Both standards require that disabling occur where a received tone falls between 2079 Hz and 2121 Hz at a signal level of between -31 dBm and 0 dBm and further indicate that a disabling circuit would meet the standard if it disabled an echo canceller on tones in the band 1900 Hz to 2350 Hz. Both standards require proper operation down to a signal to noise ratio of 11 dB. Under the G.164 standard, a disabler may validate a tone of appropriate frequency and level after 300+/-100 ms. Modems designed in accordance with the G.165 standard reverse the phase of their tone every 450 ms +/-25 ms. To meet the standard, a disabling circuit must recognize a phase variation in the range of 180.degree.+/-25.degree. while phase variations in the range of 0.degree.+/-110.degree. must not be detected.
U.S. Pat. No. 4,658,420 describes an echo canceller disabler which recognizes phase reversing tones. The disabler includes a tone detector and a separate reverse phase detector. The reverse phase detector comprises a wave shaping circuit for converting the received tone to a rectangular wave, a delay circuit, an inverter, and an EXOR circuit responsive to the output of the inverter and the wave shaping circuit. The delay circuit introduces a fixed delay so as to permit detection of a phase reversing 2100 Hz tone. If the tone received is of a different frequency, the detector may fail to recognize the phase reversal.
This invention seeks to provide a phase reversal detecting disabler using digital signal processing (DSP) techniques to detect phase reversals with a high degree of accuracy.